Rafael Nadal's hopes of successfully defending his U.S. Open title have been dealt a serious blow after the Spaniard revealed he has sustained a wrist injury during a practice session.

The world No. 2 says he will be out of competitive action for "at least two-three weeks," ruling him out of the upcoming ATP Tour 1000 events in Toronto and Cincinnati and possibly out of this season's final grand slam tournament which starts on August 25.

Nadal won both Masters events last season before beating Novak Djokovic in the final at Flushing Meadows in New York.

In a statement released by organizers of Toronto's Rogers Cup, Nadal expressed his dismay at missing out this year.

"I'm extremely disappointed that I am unable to defend my Rogers Cup title this year," said Nadal, who beat home favorite Milos Raonic in last year's final.

"I was looking forward to coming there and playing again in Toronto as I have always loved to play in Canada and had great results in the past at a very important event.

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"Unfortunately I injured myself yesterday (Tuesday) during practice and after checking with my doctors I will have to stay out of competition for at least two-three weeks. I am sorry to the tournament and to all of the fans."

The 28-year-old last missed the U.S. Open in 2012 when a knee injury forced him out of action for seven months in total.

Nadal has endured fluctuating fortunes on court this year winning four tournaments so far including the French Open in June.

His four-set win over Novak Djokovic clinched a ninth title at Roland Garros, but this was tempered by disappointments at Melbourne Park and Wimbledon.

At the Australian Open in January, Nadal injured his back in the warm up before the final and went onto to lose in four sets to Switzerland's Stanislas Wawrinka.

Normal service was resumed in Paris before Nadal exited early for the third year running at SW19.

After being dumped out in round's two and one in 2012 and 2013 respectively, Nadal made it to the last 16 before succumbing to Australian teenager Nick Kyrgios.